1. Field of the Invention
The present invention relates to storage devices that are mutually connected via a bus. More specifically the present invention pertains to a technique of identifying a desired storage device among a plurality of storage devices that are connected with one another via a bus.
2. Description of the Related Art
A variety of techniques have been proposed to select a desired storage device among a plurality of storage devices (memories) included in a memory module and carry out a writing/reading operation of data into and from the selected storage device. In a system including a plurality of storage devices connecting with a data signal line and a clock signal line via a bus, one practical technique causes identification information to be stored in advance in each storage device in some physical way, for example, using a pull-up resistance, and gains access to the desired storage device based on the identification information. In this technique, a data array to be transmitted includes a start bit representing start of the data array and an end bit representing end of the data array. Access to each storage device is specified by the combination of the start bit, the end bit, and the clock signal.
Another practical technique uses chip select signal lines in addition to the data signal line and the clock signal line and transmits a chip select signal to select a desired storage device. This technique gains access to only a storage device receiving the chip select signal among the plurality of storage devices and carries out the writing/reading operation into and from the accessed storage device.
The former technique storing the physical identification information in the storage device, for example, with a pull-up resistance, however, it is practically impossible to rewrite the identification information stored in each storage device. This technique is accordingly not suitable for recycle. The access to each storage device is specified by the combination of the start bit, the end bit, and the clock signal. In the case of accidental cutoff of the power supply during the access to the storage device, the writing operation of data can not be concluded normally.
The latter technique requires a plurality of chip select signal lines corresponding to the number of the storage devices. This undesirably requires the heavy wiring load of the signal lines and complicates the layout design. Only one chip select signal line is utilized for access, which worsens the utilization efficiency of the signal lines.